I'd like to improve my Amazon recommendations and special gold box deals by marking items that I own. I've done this in the past, and they've changed the item page so I can't find the "I own this" checkbox on the page any longer. It does appear on my recommended items page, but that's the only place I can find it. Is there any other way to mark an arbitrary item as owned?

After you mark items in the rate-it wizard - go to the "Items you've
rated" page and you should see the new items at the top of the list
with the star ratings you put in and the I-Own-It check box. AFAICT,
the wizard isn't tied to your account - it's just a tool to input
data.

So the new tool might not be exactly what you're looking for, but it's worth a shot.

The various methods discussed on that thread are not reliable. If you Amazon doesn't want us to be able to do this then there isn't much point in using one of the work-around - only to see the list disappear. The fact is that they are forcing users to either rate something or like it. Resistance is futile.
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TomJan 7 '13 at 22:05

You can mark many different items as part of your library, regardless of whether you purchased them through Amazon or not. There is also an import feature to transfer lists from other services (e.g., LibraryThing).